We know our world as an abode for life. Earth is teeming with fish, mammals, birds, reptiles, insects, and even some oddball “extremophiles” — life forms that live in extreme environments like deep inside ice at the poles or around hot springs at the bottom of the ocean. Today, the diversity of life on Earth, and its hardiness at adapting to extreme environments, prompt many scientists to think life could thrive elsewhere in our solar system, or on planets and moons around other stars in our Milky Way galaxy.

But it was only in the sixteenth century, around the time of Copernicus, that people began to see Earth as a planet at all — akin to the “wanderers” that were seen crossing the night sky against the stable background of stars. Since then, telescopic and probe-based studies of other bodies in the solar system have put our remarkable home into a broader context.

We know that Earth is the largest of the “terrestrial” planets in our solar system — the four rocky planets close to the Sun.

Earth is tilted on its axis at 23.5 degrees, which creates the seasons. When one hemisphere is tilted toward the Sun, it experiences summer. The hemisphere tilted away experiences winter. Spring and fall occur when a hemisphere is between these two extremes.

Earth is the only planet where the normal temperature range allows water to exist on its surface as a liquid. In fact, oceans cover 70 percent of Earth’s surface.

This is an extremely important factor for life as we know it.

Earth’s atmosphere is another factor that makes life possible. Consisting of 78 percent nitrogen, 21 percent oxygen, and traces of other gases, it provides protection from most of the Sun’s harmful radiation. It also protects us from most of the meteors that head for the planet; they burn up in the atmosphere before they can hit the ground.

The solid portions of Earth form three layers. The top layer is the crust, and is made of silicates — rocks like quartz, feldspar, and others. The mantle lies beneath the crust, and contains most of Earth’s mass. The mantle is made primarily of silicon, magnesium, and oxygen. The mantle wraps around a two-layered core: a partially molten outer layer of iron and nickel and an inner layer of solid iron. As Earth rotates, these layers spin at different rates, which generates our planet’s magnetic field.

Earth’s seven continents sit on large tectonic plates that float atop the mantle. Eons ago, the continents were joined together into a single land mass scientists have coined Pangaea. Starting about 200 million years ago, the plates began to drift apart, causing the continents to separate. They are still moving today, albeit extremely slowly. Earthquakes occur where tectonic plates run into each other and one plate is forced to slide underneath another.

Earth’s Moon

Earth’s partner in its yearly trek around the Sun, the Moon, is geologically dead. Dried lava fields called “maria” — Latin for seas — cover its surface, along with impact craters. The maria formed about four billion years ago, when giant asteroids punched holes in the Moon’s crust, allowing molten rock to bubble to the surface, where it cooled and hardened.

Earth and the Moon are more like a double planet than a planet and a moon. The Moon is quite large in comparison to Earth — about one-quarter of Earth’s diameter. The two gravitationally interact with each other, most famously causing Earth’s ocean tides.

The interaction has other important consequences. Over time, the Moon’s rotation has become tidally locked, so that the same side of the Moon always faces Earth. And the Moon acts to stabilize a “wobble” in Earth’s axis. Over billions of years, this has led to a much more stable climate, which was friendly to the evolution of life. The Earth-Moon interaction also slows Earth’s rotation by about two milliseconds per day per century.

The Moon probably formed in a “big whack.”

The theory arose after scientists analyzed the rocks brought to Earth by Apollo astronauts and Soviet Luna probes, as well as the readings of seismometers left on the lunar surface to record “moonquakes.”

Their studies showed that the Moon’s composition closely resembles that of Earth’s crust and mantle. From this, scientists concluded that a Mars-sized body hit Earth within a few million years of its formation. The impact vaporized much of the material in Earth’s crust and mantle and blasted them into space, forming a ring around the planet. This material quickly coalesced to form the Moon — Earth’s steady companion in its never-ending trek around the Sun.

The Distance Between Earth and the Moon is Increasing

Just like a spinning ice skater whose rotation slows as he extends his arms, the Earth-Moon distance is lengthening because Earth is spinning slower each day. The Moon’s gravitational influence is slowing Earth’s rate of rotation down by one and a half thousandths of a second every 100 years. The loss of rotational energy — angular momentum, for the physicists in the crowd — is necessarily matched by an increase in the Moon’s angular momentum, which results in a larger orbit for the Moon.

Currently, the Moon moves less than two inches a year farther away from Earth — a tiny amount, but easily measurable with modern laser-ranging devices.

Sun

By human standards, the Sun is eternal. It rises in the east every morning, sets in the west every evening, and shines brightly as it crosses the sky. Like all stars, though, the Sun undergoes constant change. Some of the changes take place over days or even minutes, others require decades, and still others require millions or billions of years.

The Sun At a Glance

Classification

G2V Main-Sequence Star

Distance from Earth

92,955,800 miles
149,597,900 km
1 Astronomical Unit (AU)

Mass

332,900 times Earth’s mass

Volume

1.3 million times Earth’s volume

Rotation Rate

25.38 Earth days (equator)

Equatorial Diameter

864,400 miles
1,391,000 km
109 times Earth’s diameter

The Sun was born about 4.6 billion years ago from the gravitational collapse of a vast cloud of gas and dust. Material in the center of the cloud was squeezed so tightly that it became hot enough to ignite nuclear fusion.

Today, the Sun continues to fuse hydrogen atoms to make helium in its core. It fuses about 600 million tons of hydrogen every second, yielding 596 million tons of helium. The remaining four million tons of hydrogen are converted to energy, which makes the Sun shine. Most of this energy is in the form of gamma-rays and X-rays. As the energy works its way to the surface — a process that takes centuries — it is absorbed by other atoms, then re-radiated at other wavelengths. When it reaches the surface, where it can escape into space, most of the energy is in the form of visible light.

The motions of the hot gas below the Sun’s surface create a powerful magnetic field. The field encircles the Sun with lines of magnetic force. These lines become entangled, forming relatively cool, dark magnetic storms on the Sun’s surface known as sunspots. Occasionally, the entangled lines “snap,” triggering enormous explosions of energy known as solar flares. Magnetic effects also pull out big streamers of hot gas from the Sun’s surface, and they heat the Sun’s thin outer atmosphere to more than one million degrees.

The number of sunspots and flares peaks every 11 years, when the Sun’s magnetic field flips over. It takes two “flips” to complete a full cycle.

The Sun will continue to burn its hydrogen for several billion years more. As it depletes the supply of hydrogen, its core will shrink and temperatures will climb high enough for it to burn helium instead. The Sun’s surface will puff up like a balloon, growing cooler, brighter, and redder, forming a red giant.

Eventually, as the Sun burns helium to form heavier elements, it will reach a critical point where fusion cannot release enough energy to form new elements, so fusion will end.

After that, the Sun will shed its outer layers, surrounding itself with a colorful bubble of gas called a planetary nebula. As the nebula dissipates, distributing carbon, oxygen, and other elements into the galaxy, only the Sun’s collapsed core will remain — a dense ball no bigger than Earth, containing about 60 percent of the Sun’s original mass. This dead remnant is called a white dwarf. Over many billions of years, the white-dwarf Sun will cool and fade from sight, leaving behind a dark cosmic ember.

What are sunspots?

Sunspots are regions on the Sun’s visible surface, or “photosphere,” where gases have been trapped by magnetic fields. The hotter material bubbling up from the Sun’s interior cannot penetrate the strong magnetic fields (about 10,000 times stronger than Earth’s), and thus are prevented from reaching the surface. These magnetic areas cool down (from about 9,800 to 6,700 degrees Fahrenheit (5,500 to 3,750 C)), so they don’t glow as brightly as the rest of the photosphere. Sunspots are actually quite bright, but appear as dark spots against their much brighter surroundings.

Sunspots have complex structures, which are caused by the geometry of the magnetic fields. The darkest area in the center, the “umbra,” is where the magnetic field is strongest. Around the edge of the sunspot, the field weakens, so this “penumbra” is a little brighter and shows radial streaks. Sometimes “light bridges” cross the umbra, like sparks jumping the gap in a spark plug.

The number of sunspots visible on the Sun’s surface varies from maximum to minimum and back again over an average period of 11 years, called the “sunspot cycle.” The most recent solar minimum occurred in December 2008. Scientists predict the next peak will occur in May 2013.

Close to the Sun

Earth snuggles closest to the Sun for the entire year during early January, less than two percent closer than the average distance of 93 million miles (150 million km). This image, from an orbiting satellite, shows “hot spots” on the Sun’s surface plus arcs of hot gas looping far into space. The image was taken in a narrow range of light that isn’t visible to the human eye. [SOHO/EIT Consortium/ESA/NASA]

Sun Power

A filament of hot gas erupts into space from the surface of the Sun in this December 6 image from NASA’s Solar Dynamics Observatory. Astronomers are still learning about the processes that create filaments and other events on the Sun’s surface. And not until the last century did they learn what powers the Sun and other stars: nuclear fusion. [NASA/SDO]

Earth’s Moon

Earth’s partner in its yearly trek around the Sun, the Moon, is geologically dead. Dried lava fields called “maria” — Latin for seas — cover its surface, along with impact craters. The maria formed about four billion years ago, when giant asteroids punched holes in the Moon’s crust, allowing molten rock to bubble to the surface, where it cooled and hardened.

Moon At a Glance

Average Distance from Earth

238,855 miles
384,400 km

Equatorial Diameter

2,159 miles
3,475 km

Length of Day

27.3 Earth days

Surface Gravity

17 percent that of Earth

Earth and the Moon are more like a double planet than a planet and a moon. The Moon is quite large in comparison to Earth — about one-quarter of Earth’s diameter. The two gravitationally interact with each other, most famously causing Earth’s ocean tides.

The interaction has other important consequences. Over time, the Moon’s rotation has become tidally locked, so that the same side of the Moon always faces Earth. And the Moon acts to stabilize a “wobble” in Earth’s axis. Over billions of years, this has led to a much more stable climate, which was friendly to the evolution of life. The Earth-Moon interaction also slows Earth’s rotation by about two milliseconds per day per century.

The Moon probably formed in a “big whack.”

The theory arose after scientists analyzed the rocks brought to Earth by Apollo astronauts and Soviet Luna probes, as well as the readings of seismometers left on the lunar surface to record “moonquakes.”

Their studies showed that the Moon’s composition closely resembles that of Earth’s crust and mantle. From this, scientists concluded that a Mars-sized body hit Earth within a few million years of its formation. The impact vaporized much of the material in Earth’s crust and mantle and blasted them into space, forming a ring around the planet. This material quickly coalesced to form the Moon — Earth’s steady companion in its never-ending trek around the Sun.

The Distance Between Earth and the Moon is Increasing

Just like a spinning ice skater whose rotation slows as he extends his arms, the Earth-Moon distance is lengthening because Earth is spinning slower each day. The Moon’s gravitational influence is slowing Earth’s rate of rotation down by one and a half thousandths of a second every 100 years. The loss of rotational energy — angular momentum, for the physicists in the crowd — is necessarily matched by an increase in the Moon’s angular momentum, which results in a larger orbit for the Moon.

Currently, the Moon moves less than two inches a year farther away from Earth — a tiny amount, but easily measurable with modern laser-ranging devices.